Synthesis, X-ray crystal structure and properties of a novel tetranuclear unsymmetrical (μ-SO 4) copper(II) complex: relevance to SO 2 fixation

Abstract

The synthesis, by fixation of SO 2, the unusual crystal structure, and the spectral and redox properties of the new μ - SO 4 2 - compound [Cu 4(TPPNOL) 2(μ-SO 4) 2](ClO 4) 2 ( 1) [HTPPNOL ( N, N, N′-tris-(2-pyridylmethyl)-1,3-diaminopropan-2-ol)] are reported. In 1, the copper(II) ions are bridged by the alkoxo oxygen atoms of the HTPPNOL ligand and by exogenous sulfate bridges. The structure of 1 consists of a centro-symmetric tetranuclear core or a “Dimer of Dimers” complex, in which a μ-O,O′ sulfate oxygen atom is further coordinated to the copper centre of another similar dinuclear unit through a μ-O,O, sulfate bridge resulting in a tetranuclear arrangement. Thus, the dinuclear units are linked by two μ-O,O sulfate bridges. The simultaneous presence of two distinct coordination modes for the sulfate group in this structure is rare and 1 represents the first coordination compound presenting μ-O,O′ and μ-O,O type structures. The SO 2 fixation was monitored by changes in the electronic spectra which indicated the formation of the intermediate hydroxo complex [Cu 2(TPPNOL)(OH) 2] +, in basic medium, which, we propose, acts as the nucleophile in the SO 2 fixation mechanism.